Combustion in a porous wall furnace

ABSTRACT

This invention is directed to a method for carrying out combustion in a furnace having porous walls with a large surface area in a combustion zone in which a low velocity oxidant stream is injected through at least one oxidant injection side of the porous walls into the combustion zone, a fuel stream through at least fuel injection side directed toward the oxidant injection side and mixing the low velocity oxidant and fuel stream to create a flame with reduced flame temperature.

This application is a continuation of prior application Ser. No.09/598,159, filed Jun. 21, 2000, now U.S. Pat. No. 6,398,546, from whichapplicant claims priority.

TECHNICAL FIELD

This invention relates generally to the field of combustion and moreparticularly to combustion in a porous wall furnace.

BACKGROUND OF THE INVENTION

In a typical process for heating and/or melting a charge such as glassor metal, certain undesirable features in the process will inevitablyoccur such as conductive heat loss from the wall of the furnace andcorrosion in the furnace wall caused by corrosive gases in the furnace.

Considerable amount of work has been done to address the concerns forovercoming these undesirable features. For example, in order to reducethe loss of heat from the furnace wall during heating and/or melting ofthe charge, more insulation may be provided. To reduce the corrosion ofthe refractory walls, certain special refractory materials may be used.Furthermore, certain techniques for premixing the fuel and oxidantbefore injecting gases into the furnace allow combustion to take placerelatively rapidly in spite of low gas injection velocity. However, tothe most part, combustion in industrial furnaces in the present state ofthe art nevertheless involves some aspects of conductive heat loss fromthe furnace and corrosion caused by corrosive gases, or some otherundesirable effects.

For example, in U.S. Pat. No. 5,609,481, a stratified atmospherecombustion is disclosed wherein both fuel and oxidant are introducedvery closely into the furnace. In this case, a charge proximal stratumis established between the charge and the combustion gas emanating fromone or more burners that are oriented above the charge. Thecharge-proximal stratum has a different oxidative effect on the chargethan does the combustion gases. This method requires both fuel andoxidant to be introduced at very low velocities into a furnace, whichtends to cause the fuel and oxidant mixing to be very slow, resulting inpoor combustion, like sooty flame.

In another example, U.S. Pat. No. 5,076,779 provides for a segregatedzoning combustion. This combustion method separates the oxidant mixingzones and fuel reaction zones in a combustion zone to dilute oxidant andcombust fuel under conditions which reduces the NOx formation. Thismethod requires that the oxidant be injected in high velocity anddiluted with the furnace gas, thus requiring a high oxygen supplypressure.

Accordingly, it is an object of this invention to provide a furnacesystem that will enable effective heating and/or melting of a charge sothat the heat loss of the system is dramatically reduced, reducing thecorrosion of the refractory walls and injecting the oxidant at a lowvelocity while mixing the oxidant with the fuel or furnace gases at afast rate.

OBJECTS OF THE INVENTION

It is an object of the invention to provide a method for combustion thatreduces the heat loss in the combustion process.

It is another object of this invention to provide a method forcombustion that reduces the corrosion in the refractory walls of thefurnace.

It is yet another object of this invention to provide a method forcombustion that introduces an oxidant at a very low velocity, and whichmixes the fuel or furnace gas at a fast rate.

SUMMARY OF THE INVENTION

This invention is directed to a method for carrying out combustion in afurnace having porous walls. The furnace has a combustion zonecontaining an atmosphere of furnace gases. A low velocity oxidant streamis injected through at least one oxidant injection side of the porouswalls into the combustion zone. A fuel stream is injected through atleast one fuel injection side directed toward the oxidant injection sidein the combustion zone. Furnace gases are separately mixed with at leastan oxidant stream and a fuel stream to produce at least a separatediluted oxidant-furnace gas mixture and a fuel furnace gas mixture. Theoxidant-furnace gas mixture is mixed with the fuel-furnace gas mixtureto create a diluted fuel-oxidant-furnace gas mixture.

In an alternative embodiment, a low velocity fuel stream is injectedthrough at least one fuel injection side of the porous walls into thecombustion zone, and the oxidant stream is injected through at least oneoxidant injection side directed toward the fuel injection side of thecombustion zone.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages will occur to those skilled inthe art from the following description of preferred embodiments and theaccompanying drawings, in which:

FIG. 1 is a drawing of the porous wall furnace of this invention showinga stratified atmosphere combustion for melting glass therein;

FIG. 2 is a detailed drawing of porous wall elements wherein the oxidantor purge gas is to pass through the porous wall element;

FIGS. 3a and 3 b are schematic drawings of the position in the porouswall furnace which show the metal charge using the stratified atmospherecombustion method; and

FIGS. 4a and 4 b are schematic drawings of the position in the porouswall furnace which show the placement of the oxidant, fuel, flue and ametal charge using the dilute oxidant combustion method.

FIG. 5 is a schematic drawing showing the porous wall furnace positionwhere the placement of the oxidant, fuel, flue and a metal charge usinga fuel stream impinging on the porous wall with oxidant flow.

DETAILED DESCRIPTION OF THE INVENTION

In this invention, oxidant is introduced into a furnace through a porouswall at low velocity, and fuel is introduced into the furnace from oneor more of the furnace walls. In the preferred embodiments, fuel isintroduced into the furnace from the adjacent wall(s), substantiallyparallel to the porous surface of the furnace for combustion to takeplace. The fuel is introduced at a distance from the porous wall so thatoxidant mixes with a recirculating flow of furnace gases prior tocombustion with fuel (i.e., to achieve dilution of oxidant for low NOxemissions). When more rapid combustion is desired, several fuel jets canbe directed toward the porous walls so that the fuel jets, afterdilution through furnace gas entrainment, impinge on the porous wallsurfaces.

FIG. 1 shows an application of the porous wall combustion process for aglass melting furnace 10. Oxidant 30, preferably oxygen, is introducedinto the furnace through the furnace porous layer 20 of furnace roof 26or crown. The furnace roof 26 may be constructed by joining wallelements shown in FIG. 2. Fuel 34 is injected from furnace side wall 28for combustion forming flames 36 in the combustion zone 40. The chargeor raw material for glass making is introduced into the furnace from anend of the furnace (not shown) and forms a pool of molten glass 48 overthe bottom wall 50 of the furnace. In a preferred embodiment, theoxidant stream 30 is provided into the combustion zone 40 from thefurnace roof 26. The oxidant stream is then mixed with recirculatingfurnace gases, causing the concentration of oxygen in the oxidant streamto dilute. A preferred combustion process is through a stratifiedatmosphere combustion, as shown in FIG. 1, where a second source ofoxidant 38 is introduced through a lower portion of sidewall 34. Theintroduction of the second source of oxidant forms an oxygen rich zone44 between the combustion zone 40 and the charge 48.

Fuel lances are located in the furnace side walls and the flow ratescontrolled to properly distribute the heat generation within the furnaceand to cause furnace gas recirculation. The pore size and thickness ofthe roof can be adjusted to control the flow of oxidant per unit surfacearea and the pressure requirement while keeping the conductive heat lossthrough the wall negligible.

Combustion in the porous wall furnace of this invention involves theprovision of oxidants (via an oxidant stream) at a low injection rate.The oxidant is heated by passing through the hot furnace wall andtypically introduced at a specific flow rate less than 600 SCFH/ft²,corresponding to an actual oxidant velocity at the porous wall hotsurface temperature of less than about 1 ft/sec. Air, oxygen enrichedair, or pure oxygen can be used as oxidant. A low oxidant specific flowrate is preferred to reduce the pressure drop across the porous wall andto avoid potential hot spot on the porous wall hot face when using animpinging fuel stream.

In order to eliminate the wall conduction heat loss, the sensible heatof the heated oxidant must be greater than the conductive heat fluxthrough the porous wall without passing the oxidant. Since an importantobjective is to minimize furnace wall losses, it is preferable to flowthe oxidant through most of the available surface areas of the furnaceroof and walls (at least 20% of the available roof and wall surfaceareas of the combustion space). In the preferred embodiment, the entirefurnace roof is used and the specific flow rate is less than 150SCFH/ft². The oxidant that flows from the porous roof purges the hotface of the roof and helps to prevent corrosive vapors from coming incontact with the roof refractory material.

Comparatively, in the conventional post-mix burner, both the fuel andoxygen streams are injected into the combustion zone of the furnace atsufficiently high velocities to achieve rapid mixing of the fuel andoxidant. Typical velocity of combustion air is 30 to 200 ft per second.

There are many porous refractory materials suitable for use in thepresent porous wall combustion process. They include refractory wallsmade of fibers, sintered particles and foams. As shown in FIG. 2, ametallic rear chamber 264 is preferred to supply and pass oxidantuniformly through a porous wall element 266. FIG. 2 shows alternativedesign made of a refractory material without a metallic rear chamber. Anon-porous refractory rear body is joined to a front porous refractoryelement with a gas distribution passage. Oxidant or purge gas 260 isintroduced through the rear 266 of the furnace and into a rear chamber264 prior to uniformly passing through the porous wall element 226.Porous roofs or walls may be constructed by connecting the modularporous wall element 226. The minimum oxidant flow rate to eliminate thewall conduction heat loss can be estimated from the steady state heatloss of the porous material per unit surface area without any gas flow,Qo,

Qo=cV(Tw−To)

where c is the specific heat of the oxidant, V is the volume flow rateof the oxidant per unit surface area, and Tw and To are the hot facetemperature of the wall and the cold face temperature of the wall,respectively. Typical specific wall heat loss through a glass furnaceroof without insulation is in a range between 500 to 1,500 Btu/hr/ft².For example, 12 inch thick porous silica bricks may be used for glassfurnace crown. Assuming that Qo=1000 Btu/hr/ft² at Tw=2900° F. andTo=300° F., and c=8.25 Btu/lb mol/F=0.0217 Btu/F/ft³ at 70° F., V=18SCFH (ft³ of gas at 70° F. per hr per ft²) which provides a hot face gasvelocity of 0.03 ft/sec at 2900° F.

The fuel for mixing with the oxidant for combustion can be introducedwith and without a portion of overall oxidant. The fuel injection methoddepends on the particular applications. For the stratified atmosphericcombustion process (see, U.S. Pat. No. 5,609,481), a preferred method isto introduce fuel at low velocity so as to flow fuel near the poroussurface and distribute over the entire surface area. One embodiment isshown in FIG. 1. Further embodiments of combustion arrangement using thestratified atmospheric combustion process for glass melting, metalheating and other furnaces are shown in FIGS. 3a and 3 b.

In FIG. 3a, the oxidant 330 is introduced from the furnace roof 326 ontoa gas cavity 392 before passing through the porous wall layer 320 ontothe combustion zone 340. Fuel 334 through multiple fuel nozzles (notshown) is injected toward the porous wall through one sidewall 328 ofthe furnace 310 into the combustion zone 340 to distribute fuel evenlyover most of the porous wall area to form wide flames 336 forcombustion. Flue gas 378 is released from the furnace through the upperportion of sidewall 329. Fuel jets may be directed toward, parallel to,or away from the porous wall in order to control the flame shape andcoverage. A protective atmosphere in the form of a non-combustible gasis formed in the furnace between the combustion zone 340 and the charge348. The protective atmospheric gas 370 is injected through the bottomportion of sidewall 328 to form a protective atmosphere zone 372. Theprotective atmospheric gases may include nitrogen and/or argon.

Another preferred embodiment of the present invention using thestratified atmospheric combustion is provided in FIG. 3b. Oxidant 330 isinjected through a side wall extension 327, which then passes throughthe gas cavity 392 before passing through porous wall layer 320 into thefurnace 310. Fuel 334 is injected through sidewall 328 into furnace 310forming a flame 336 in combustion zone 340 therein. Flue gas 378 isreleased through the furnace roof 326. A protective atmosphere in theform of a noncombustible gas zone 372 is formed about the charge 348 bythe injection of protective atmospheric gas 370 through a lower portionof the sidewall 328.

For the dilute oxygen combustion process using the basic concept ofsegregated zoning combustion (see, U.S. Pat. No. 5,076,779), the fuelmay be injected parallel to the porous surface at higher than 100ft/sec, preferably higher than 120 ft/sec, at some distance away fromthe porous surface so as to create recirculating flow of furnace gasesover the porous surface and to dilute the oxygen concentration ofoxidant before reacting with fuel. This type of combustion isrepresented by FIGS. 4a and 4 b.

A dilute oxygen combustion (DOC) scheme that injects oxygen through aporous wall layer is shown in FIG. 4. Fuel 434 is injected throughsidewall 428 into the furnace 410. Fuel 434 forms a jet flame 436 (or afuel reaction zone) and its high injection velocity caused a strongfurnace gas recirculation flow pattern 490 for dilution of oxidant priorto combustion in flame 436. Oxidant 430 is injected through a gas cavity492 and through porous layer 420 into the combustion zone 440.

In an alternative preferred embodiment of the present invention usingDOC, fuel is injected from the furnace roof and the oxidant is injectedfrom the sidewall. In FIG. 4b, two separate sources of fuel 434 areinjected through the roof 426 of the furnace 410. Separate sources ofoxidant 430 are injected for more even and complete combustion. Oxidant430 is injected through gas cavity 492 for greater even distribution ofgas flowing into the porous layer 420 into the combustion zone 440. Fuel434 forms jet flame 436 (or a fuel reaction zone) and the high fuelinjection velocity causes a strong furnace gas recirculation flowpattern 490 for diluting the oxidant before combusting in flame 436.Charge 448 is heated both by furnace radiation and gas convection. Fluegas 478 is released through sidewall 428.

As mentioned previously, jets of fuel streams can be injected toward theporous wall with oxidant emanating from the surface in someapplications. FIG. 5 is a schematic drawing of the position in theporous wall furnace that shows the placement of the oxidant, fuel, flueand a metal charge using a fuel stream impinging on the porous wall withoxidant flow. Fuel 534 is injected through sidewall 528 into the furnace510. Fuel 534 forms a jet 536 that impinges on the surface of the porouswall 520. Multiple fuel jets may be placed to distribute fuel evenlyover the porous wall. Oxidant 530 is injected through a gas cavity 592.Flue gas 578 is released through sidewall 528.

Rapid combustion may take place at or near the porous wall in thisarrangement. A concern on this arrangement is potential overheating ofthe porous wall, especially near the fuel jet impingement area. In thiscase, careful placement of the fuel injection ports as well as a properselection of the oxygen flux through the porous wall are required. Ifthe impinging fuel jet contains a high combustible gas concentrationnear the impingement point, the net oxygen flux through the porous wallshould be limited to less than 200 SCFH, preferably less than 100 SCFH.The net oxygen flux is calculated by multiplying the oxidant fluxthrough the porous wall by the fractional oxygen volume concentration inthe oxidant. By limiting the oxygen flux, the maximum heat release atthe surface of the porous wall is controlled to about 100,000 Btu/hr/ft²at an oxygen flux of 100 SCFH. Because of the cooling of the poroussurface by radiant heat transfer, excessive heating can be avoided andlow NOx emissions are achieved without dilution with furnace gases. Ifthe impinging fuel jet contains little combustible gases, then higheroxygen flux is beneficial due to the cooling effect of oxidant.

An important variation of the present porous wall combustion process isthe use of a ceramic membrane material in place of a porous material soas to enable the oxygen or enriched oxygen to pass into the furnacewhile feeding air on the other side of the membrane material. U.S. Pat.No. 5,888,272 describes processes using ceramic membranes in acombustor. In order to limit the maximum temperature of the membranematerial, a membrane wall furnace chamber may be added to the mainfurnace with flue gas recirculation between the main furnace and themembrane wall furnace chamber. For example, an integrated membrane airseparation may be used in conjunction with the dilute oxygen combustionprocess. This process utilizes a ceramic membrane air separation chamberon one side of the furnace to supply oxygen or enriched oxygen into thefurnace.

In an alternative embodiment, fuel is used as the purge gas in place ofoxidant. In this way, the fuel stream is introduced into the porous wallat a low velocity, and the oxidant is introduced into the furnace fromthe adjacent wall(s), toward the porous surface of the furnace forcombustion.

In typical combustion of natural gas with air, the volume flow rate ofair is about ten times of that of fuel. Thus, fuel provides a relativelysmall heat capacity to counter the conductive heat loss. For oxy-fuelcombustion, the volume ratio of oxygen to natural gas is about 2 to 1,and the use of fuel as a purge gas may be important for someapplications. For example, it is advantageous to use fuels withrelatively low heating values, such as blast furnace gas and coke oven ogas, as purge gas. For combustion of a typical blast furnace gas, about2 volume of fuel is required to combust one volume of combustion air.

The use of both fuel and oxidant as separate purge gases are alsopossible. However, it is difficult to achieve good mixing forcombustion. Premixing of fuel and oxidant and combusting at the surfaceof the porous refractory material is known. However, the velocity of thepremixed gas must be high enough to prevent the flash back.

Premature ignition within the porous material would result inoverheating or melting of the porous material. This is particularly aproblem when oxygen enrich air or oxygen is used. As a result, it isundesirable to premix the fuel and oxidant unless the velocity of thepremix gas is introduced at a sufficiently high velocity to preventflash back.

Other potential problems of using a hydrocarbon fuel as a purge gas isthe formation of soot and reaction with porous refractory materials.Silicon carbide and other materials are available for such applications.In order to prevent soot formation, a mixture of fuel and recycled fluegas may be used, especially for oxy-fuel combustion applications. Inthis case endothermic reaction of methane with carbon dioxide and watermay provide a special benefit of recovering the heat as chemical energy.

Specific features of the invention are shown in one or more of thedrawings for convenience only, as each feature may be combined withother features in accordance with the invention. Alternative embodimentswill be recognized by those skilled in the art and are intended to beincluded within the scope of the claims.

What is claimed is:
 1. A method for carrying out combustion in a glassmelting furnace having a porous roof comprising: a) providing acombustion zone containing an atmosphere of furnace gases; b) providinga charge on the bottom of the furnace; c) injecting a low velocityoxidant stream through at least one oxidant injection side of the porousroof into the combustion zone at a flow rate of less than 600 SCFH/ft²;d) injecting a second source of oxidant through a lower portion of asidewall and creating an oxygen rich zone between the combustion zoneand the charge; e) injecting a fuel stream through at least one fuelinjection side; f) mixing the low velocity oxidant, the second source ofoxygen and fuel stream in the combustion zone to create a flame withreduced temperature.
 2. The method of claim 1 comprising passing theoxidant stream through the roof of the furnace and the fuel streamthrough at least one side wall of the furnace.
 3. The method of claim 1further comprising mixing said furnace gas with the oxidant stream in amixing zone to produce a diluted oxidant-furnace gas mixture prior tomixing the fuel to create a flame.
 4. The method of claim 1 furthercomprising mixing said furnace gas with the fuel stream to produce adiluted fuel-furnace mixture prior to mixing with the oxidant to createa flame.
 5. A method for carrying out combustion in a furnace havingporous walls comprising: a) providing a combustion zone containing anatmosphere of furnace gases; b) injecting a low velocity oxidant streamthrough at least one oxidant injection side of the porous walls into thecombustion zone at a new oxygen flux of less than 200 SCFH/ft²; c)injecting a fuel stream through at least one fuel injection sidedirected toward the oxidant injection side in the combustion zone andimpinging on the porous surface; d) mixing the fuel and oxidant tocreate a flame on or near the surface of the porous wall; and e)radiating heat from the porous wall surface to furnace heat load toreduce the surface temperature while transferring heat.
 6. The method ofclaim 5, wherein the low velocity oxidant stream is injected through atleast one oxidant injection side of the porous walls into the combustionzone at a new oxygen flux of less than 100 SCFH/ft².
 7. The method ofclaim 5 comprising passing the oxidant stream through the roof of thefurnace and the fuel stream through at least one side wall of thefurnace.
 8. The method of claim 5 further comprising mixing said furnacegas with the oxidant stream in a mixing zone to produce a dilutedoxidant-furnace gas mixture prior to mixing the fuel to create a flame.9. The method of claim 5 further comprising mixing said furnace gas withthe fuel stream to produce a diluted fuel-furnace mixture prior tomixing with the oxidant to create a flame.